Project Summary This proposal is for a new R21, and we are responding to the NIAID's Interest to Highlight High-Priority Zika virus (ZIKV) Research Areas (NOT-AI-16-026). ZIKV is a rapidly emerging flavivirus that is spread primarily by the mosquito vector, Aedes aegypti. The current ZIKV outbreak in South America has been associated with a 20-fold increase in the rate of babies born with microcephaly and has also been linked with the development of Guillain-Barre syndrome. The regions involved in the current outbreak are known to be endemic for another flavivirus, dengue virus (DENV). This indicates that a large percentage of the population may have preexisting DENV immunity. It has been shown that preexisting immunity to DENV can enhance infection with other closely related flaviviruses. ZIKV is genetically very close to DENV. The purpose of this study is to identify potential enhancement of ZIKV infection by antibodies recognizing DENV. As the World Health Organization considers the current ZIKV epidemic and related microcephaly association an international public health emergency, investigations into ZIKV infection and behavior in human immune cells, with and without preexisting flaviviral immunity, are a priority area of research. Our central hypothesis is that antibodies recognizing DENV can complex with ZIKV and that this immune complex is able to infect monocytes and other Fc receptor-bearing cells, which would result in higher viremia. Specifically, Aim 1 will establish a model of antibody-dependent enhancement using ZIKV infection and sera from South American DENV patients, collected prior to the current ZIKV outbreak. Aim 2 will characterize the role preexisting DENV immunity may play in ZIKV infection of neural cells. We will examine avidity and affinity between ZIKV and DENV antibodies; evaluate ZIKV infection in primary human immune cells and neural cells in the presence of antibodies recognizing DENV 1-4; and examine cytokine and chemokine production by ZIKV infected cells with and without anti-DENV immune sera. Together, these aims will provide a more comprehensive picture of ZIKV infection in the presence of preexisting DENV immunity.